PROJECT SUMMARY Adolescent-onset depression is a particularly virulent form of depression due to increased resistance to antidepressant treatment. Furthermore, the manifestation of depression in adolescence and young adulthood is associated with higher risk for depression recurrence across the lifespan. As a result, there is a dire need for the development of novel antidepressants that are suitable for the adolescent brain. Adolescence represents a critical period of neurodevelopment. It is defined by structural and synaptic maturation and reorganization within the prefrontal cortex (PFC). During this time, dendritic spines, the primary site of excitatory synapses, are pruned, refined, and stabilized. Research in animal models suggests that structural stability confers resilience to neuropsychiatric diseases, such as depression. Research using conditional knockout mice indicates that one critical regulator of the neural maturation and stabilization in adolescence is the ?1-integrin receptor. The ?1-integrin signaling pathway inhibits Rho Kinase-II (ROCKII) activity to promote stabilization of the actin cytoskeleton. I have shown that the ROCKII inhibitor, fasudil, has antidepressant-like efficacy in adolescent, but not adult, mice. Fasudil also prunes dendritic spines while increasing synaptic marker expression in the adolescent PFC, suggesting that it enhances typical dendritic spine pruning and strengthens synapses. I hypothesize that fasudil converges with endogenous ?1-integrin- mediated signaling events during adolescence to exert its structural and therapeutic-like benefits. Aim 1 will measure the expression and activity levels of proteins downstream of the ?1-integrin receptor in the healthy developing PFC, a critical step in determining the feasibility of therapeutically targeting the ?1-integrin signaling cascade in adolescents. Next, Aim 2 will use viral-mediated gene silencing, confocal fluorescence microscopy, and 3D dendritic spine reconstruction to test the hypothesis that fasudil?s beneficial impact on the maturation and pruning of dendritic spines in the PFC during adolescence is due to convergence with endogenous ?1-integrin-mediated signaling and synergistic inhibition of ROCKII. Aim 3 will evaluate the antidepressant-like efficacy of enhancing signaling of the ?1-integrin cascade and will test the hypothesis that the behavioral efficacy of fasudil is also attributable to convergence with endogenous ?1-integrin-mediated signaling to inhibit ROCKII. I will also assess the antidepressant-like efficacy of DPH, a novel activator of Arg kinase, a primary ?1-integrin substrate in the PFC. Lastly, I will test whether site-selective ?1-integrin knockdown in the adolescent PFC induces a depressive-like phenotype. Positive outcomes from these experiments will indicate that ?1-integrin signaling events may be a viable target in the development of novel intervention strategies for adolescent-emergent depression and will identify ?1-integrin-mediated signaling as a resiliency factor in mood regulation in adolescence.